This invention relates to remote manipulators and, more particularly, to remote manipulators for inspecting and repairing nuclear boiler generators.
A typical nuclear boiler generator comprises a nuclear reactor core surrounded by a vertically oriented cylindrical shell, with hemispherical caps at each end of the cylindrical shell. In order to collect heat generated by the nuclear core, a fluid is circulated through a large number of vertical tubes interspersed through the core. Each tube carries fluid up from the lower hemisphere through the core, then arches above the core to carry the fluid back down through the core then arches above the core to carry the fluid back down through the core to the lower hemisphere. The tubes begin and end at a horizontal surface referred to as a "tube sheet", which forms the ceiling of the lower hemisphere.
Due to the nature of nuclear reactor operation, the circulating fluid becomes radioactive. A secondary fluid, known as "feedwater", is circulated around the exterior of the tubes above the reactor core, in order to transfer heat from the circulation fluid to the feedwater. The non-radioactive feedwater is then used to drive a generating turbine. Occasional leaks from the circulating tubes allow radioactive fluid to mix with feedwater, however, which requires the repair of the leaking tube in order to minimize the release of radioactivity to the exterior environment. Typically, because the number of tubes intersperced in the core exceeds the number needed to efficiently transfer heat, repair is effected by sealing both ends of the leaking tube at the tube sheet, so that fluid no longer circulates therethrough.
In order to seal both ends of the tube at the tube sheet, manholes are provided in the lower hemisphere for access to the tube sheet. However, the interior of the hemisphere maintains high residual radioactivity even when drained of fluid, which limits the amount of time an individual may be allowed to spend with the hemisphere. In a typical welding operation to plug a single tube, which may take several hours to complete by hand, a worker is allowed to spend no more than a few minutes inside the hemisphere, and must immediately thereafter discard a protective suit costing many hundreds of dollars. Accordingly, it would be advantageous to perform operations on the tube terminii within the hemisphere without requiring the presence of working personnel.
There are several types of mechanisms known in the prior art that attempt to provide a solution to this problem, but all of these devices exhibit difficulties which limit their usefulness. One type of device is the walking platform. This device provides a horizontal platform bearing various tools and inspection systems, held against the tube sheet by fingers which extend into tube ends. The fingers provide horizontal stability and, in some instances, vertical support as well. Typically, the mechanisms allow withdrawal of a finger and a translation thereof across the tube sheet to a remote tube and, followed by securing the finger in the remote tube end, withdrawal, translation and replacement of subsequent fingers, followed finally by translation of the horizontal platform. Platform systems are, of course, limited as to their ultimate weight by the strength of the tube sheet and tube ends to resist vertical and horizontal motion of the inserted fingers. This problem is particularly acute when fingers are translated to a remote tube end, at which point the leverage on the tube end from the distant platform is greatest. Such platform systems are exemplified by U.S. Pat. No. 3,913,452; 3,913,752; 4,004,698; 4,018,344; 4,018,345; 4,018,346; 4,070,561; 4,074,814; and 4,193,735. These systems additionally exhibit an inability to repair tube ends adjacent a vertical barrier within the hemisphere, such as the hemisphere itself or a central tube sheet support column.
A second type of system, exemplified by U.S Pat. No. 3,934,731, is a moveable crane type apparatus with the main horizontal travel beam attached to various tube ends for horizontal and vertical support. This device, likewise, offers the possibility of damage to the tube ends supporting the travel beam, due to leverage on the travel beam caused by the crane extended therefrom. Moreover, tube ends covered by the travel beam cannot be sealed.
A third type of system provides a vertical support column between the tube sheet and the bottom of the hemisphere, upon which is supported a swinging boom carrying the work platform. In some cases, the vertical column has been supported by connection to the central tube sheet support column rather than the tube sheet itself, or may be clamped to the manhole rim rather than resting on the bottom most point of the hemisphere. These systems are typically much bulkier than the platform systems and inhibit access of personnel to the hemisphere interior during operation or installation for troubleshooting purposes. Moreover, they exhibit an inability to reach tube ends adjacent the top of the vertical column. Also, repair operations which require the use of force, such as milling, tax the stability of the swinging horizontal boom. Such systems are typified by U.S. Pat. No. 4,192,053; 4,200,424; 4,205,939; 4,205,940; 4,212,583; and 4,222,699.
U.S. Pat. No. 4,173,060 discloses a horizontal swinging boom mounted on a telescoping diagonal support which extends into the hemisphere from a stationary mounting outside the manhole. This system limits the mass of the work platform on the boom in order to minimize the leverage force exerted on the telescoping support, and completely blocks access through the manhole.
U.S. Pat. No. 4,158,415 and 4,303,368 disclose a remote segmented arm for installation of repair tools adjacent a tubesheet. The arm is coupled to a base 66 at horizontal sliding joint 72. the arm is incapable of remote installation. The arm does not support the tool head against the tubesheet, but merely transports it there, where it is locked to tubes for support. In the '368 patent, the arm delivers the tool head to a traversing mechanism which requires initial placement against the tubesheet by hand. Neither power nor control signals are delivered through the arm, which disconnects from the tool head prior to performance of work. Thus, the arm is able only to remove the tool head to change tools outside the manhole, and cannot carry multiple tools or relocate the tool head to a separate location.